Expression of GTPase-deficient G alpha 16 inhibits Swiss 3T3 cell growth

The multimodal action of G alpha q in coordinating growth and homeostasis in the Drosophila wing imaginal disc

Background

G proteins mediate cell responses to various ligands and play key roles in organ development. Dysregulation of G-proteins or Ca ²⁺ signaling impacts many human diseases and results in birth defects. However, the downstream effectors of specific G proteins in developmental regulatory networks are still poorly understood.

Methods

We employed the Gal4/UAS binary system to inhibit or overexpress Gαq in the wing disc, followed by phenotypic analysis. Immunohistochemistry and next-gen RNA sequencing identified the downstream effectors and the signaling cascades affected by the disruption of Gαq homeostasis.

Results

Here, we characterized how the G protein subunit Gαq tunes the size and shape of the wing in the larval and adult stages of development. Downregulation of Gαq in the wing disc reduced wing growth and delayed larval development. Gαq overexpression is sufficient to promote global Ca ²⁺ waves in the wing disc with a concomitant reduction in the Drosophila final wing size and a delay in pupariation. The reduced wing size phenotype is further enhanced when downregulating downstream components of the core Ca ²⁺ signaling toolkit, suggesting that downstream Ca ²⁺ signaling partially ameliorates the reduction in wing size. In contrast, Gαq -mediated pupariation delay is rescued by inhibition of IP ₃ R, a key regulator of Ca ²⁺ signaling. This suggests that Gαq regulates developmental phenotypes through both Ca ²⁺ -dependent and Ca ²⁺ -independent mechanisms. RNA seq analysis shows that disruption of Gαq homeostasis affects nuclear hormone receptors, JAK/STAT pathway, and immune response genes. Notably, disruption of Gαq homeostasis increases expression levels of Dilp8, a key regulator of growth and pupariation timing.

Conclusion

Gαq activity contributes to cell size regulation and wing metamorphosis. Disruption to Gαq homeostasis in the peripheral wing disc organ delays larval development through ecdysone signaling inhibition. Overall, Gαq signaling mediates key modules of organ size regulation and epithelial homeostasis through the dual action of Ca ²⁺ -dependent and independent mechanisms.

G Protein-Coupled Receptors in cancer: biochemical interactions and drug design

G Protein-Coupled Receptors (GPCRs) share the same topology made of seven-transmembrane segments and represent the largest family of membrane receptors. Initially associated with signal transduction in differentiated cells, GPCRs and heterotrimeric G proteins were shown to behave as proto-oncogenes whose overexpression or activating mutations confer transforming properties. The first part of this review focuses on the link between biochemical interactions of a GPCR with other receptors, such as dimerization or multiprotein complexes, and their oncogenic properties. Alteration of these interactions or deregulation of transduction cascades can promote uncontrolled cell proliferation or cell transformation that leads to tumorigenicity and malignancy. The second part concerns the design of drugs specifically targeting these complex interactions and their promise in cancer therapy.

Activation of STAT3 by Gαs Distinctively Requires Protein Kinase A, JNK, and Phosphatidylinositol 3-Kinase

Signal transducer and activator of transcription 3 (STAT3) can be stimulated by several G(s)-coupled receptors, but the precise mechanism of action has not yet been elucidated. We therefore examined the ability of Galpha(s)Q226L (Galpha(s)QL), a constitutively active mutant of Galpha(s), to stimulate STAT3 Tyr705 and Ser727 phosphorylations in human embryonic kidney 293 cells. Apart from Galpha(s)QL, the stimulation of Galpha(s) by cholera toxin or beta2-adrenergic receptor and the activation of adenylyl cyclase by forskolin, (Sp)-cAMP, or dibutyryl-cAMP all promoted both STAT3 Tyr705 and Ser727 phosphorylations. Moreover, the removal of Galpha(s) by RNA interference significantly reduced the beta2-adrenergic receptor-mediated STAT3 phosphorylations, denoting its capacity to regulate STAT3 activation by a G protein-coupled receptor. The possible downstream signaling molecules involved were assessed by using specific inhibitors and dominant negative mutants. Induction of STAT3 Tyr705 and Ser727 phosphorylations by Galpha(s)QL was suppressed by inhibition of protein kinase A, Janus kinase 2/3, Rac1, c-Jun N-terminal kinase (JNK), or phosphatidylinositol 3-kinase, and a similar profile was observed in response to beta2-adrenergic receptor stimulation. In contrast to the Galpha16-mediated regulation of STAT3 in HEK 293 cells (Lo, R. K., Cheung, H., and Wong, Y. H. (2003) J. Biol. Chem. 278, 52154-52165), the Galpha(s)-mediated responses, including STAT3-driven luciferase activation, were resistant to inhibition of phospholipase Cbeta. Surprisingly, Galpha(s)-mediated phosphorylation at Tyr705, but not at Ser727, was resistant to inhibition of c-Src, Raf-1, and MEK1/2 as well as to the expression of dominant negative Ras. Therefore, as with other Galpha-mediated activations of STAT3, the stimulatory signal arising from Galpha(s) is transduced via multiple signaling pathways. However, unlike the mechanisms employed by Galpha(i) and Galpha(14/16), Galpha(s) distinctively requires protein kinase A, JNK, and phosphatidylinositol 3-kinase for STAT3 activation.

Transcriptional activation of c-Fos by constitutively active Galpha(16)QL through a STAT1-dependent pathway

Hematopoietic restrictive Galpha(16) has long been known to stimulate phospholipase Cbeta (PLCbeta) and induce mitogen-activated protein kinase (MAPK) phosphorylation. Recently, we have demonstrated that Galpha(16) is capable of inducing the phosphorylation and transcriptional activation of transcription factors, such as signal transducer and activator of transcription 3 (STAT3) and nuclear factor kappaB (NFkappaB). However, the downstream signaling regulation by Galpha(16) has not yet been documented. In the present study, we have determined the signaling mechanism by which constitutively active Galpha(16) mediates c-Fos transcriptional activation in human embryonic kidney (HEK) 293 cells. Overexpression of constitutively active Galpha(16), Galpha(16)QL, resulted in the stimulation of c-Fos transcriptional activation in HEK 293 cells. The participation of PLCbeta, c-Src/Janus kinase 2 (JAK2) and extracellular signal-regulated kinase (ERK) signaling pathways in Galpha(16)QL-induced c-Fos transcriptional activation was demonstrated by the use of their specific inhibitors. However, c-Jun N terminal kinase (JNK), p38 MAPK and phosphatidylinositol-3 kinase (PI3K) were not required. Interestingly, the dominant negative mutant of STAT1, but not STAT3, suppressed c-Fos transcriptional activation induced by Galpha(16)QL, implying that STAT1 was involved in this signaling mechanism. To further examine the role of STAT1 in the signaling pathway of Galpha(16), we demonstrated that Galpha(16)QL was able to induce STAT1 activation. Also, stimulation of adenosine A1 receptor-coupled Galpha(16) was shown to induce ERK and STAT1 phosphorylations in a concentration-dependent manner. Using selective inhibitors, PLCbeta, c-Src/JAK and ERK, but not JNK, p38 MAPK and PI3K, were shown to be involved in Galpha(16)QL-induced STAT1 activation. Collectively, our results demonstrate for the first time that stimulation of Galpha(16) can lead to STAT1-dependent c-Fos transcriptional activation via PLCbeta, c-Src/JAK and ERK pathways.

Differential Effects of Gqα, G14α, and G15α on Vascular Smooth Muscle Cell Survival and Gene Expression Profiles

Gqalpha family members (Gqalpha, G11alpha, G14alpha, and G15/16alpha) stimulate phospholipase Cbeta (PLCbeta) and inositol lipid signaling but differ markedly in amino acid sequence and tissue distribution predicting unappreciated functional diversity. To examine functional differences, we compared the signaling properties of Gqalpha, G14alpha, and G15alpha and their cellular responses in vascular smooth muscle cells (VSMC). Constitutively active forms of Gqalpha, G14alpha, or G15alpha elicit markedly different responses when introduced to VSMC. Whereas each Galpha stimulated PLCbeta to similar extents when expressed at equal protein levels, Gqalpha and G14alpha but not G15alpha initiated profound cell death within 48 h. This response was the result of activation of apoptotic pathways, because Gqalpha and G14alpha, but not G15alpha, stimulated caspase-3 activation and did not alter phospho-Akt, a regulator of cell survival pathways. Gqalpha and G14alpha stimulate nuclear factor of activated T cell (NFAT) activation in VSMC, but Galpha-induced cell death seems independent of PKC, InsP(3)/Ca(2+), and NFAT, in that pharmacological inhibitors of these pathways did not block cell death. Gene expression analysis indicates that Gqalpha, G14alpha, and G15alpha each elicit markedly different profiles of altered gene sets in VSMC after 24 h. Whereas all three Galpha stimulated changes (> or =2-fold) in 50 shared mRNA, Gqalpha and G14alpha (but not G15alpha) stimulated changes in 221 shared mRNA, many of which are reported to be pro-apoptotic and/or involved with TNF-alpha signaling. We were surprised to find that each Galpha also stimulated changes in nonoverlapping Galpha-specific gene sets. These findings demonstrate that Gqalpha family members activate both overlapping and distinct signaling pathways and are more functionally diverse than previously thought.

Characterization of CHO Cells Stably Expressing a Gα16/zChimera for High Throughput Screening of GPCRs

G protein-coupled receptors (GPCRs) are important therapeutic targets for drug discovery. The identification and characterization of new ligands ideally requires the use of high throughput assays that are applicable to all GPCR subtypes. To circumvent the problem of different GPCRs coupling to distinct intracellular second messenger pathways, we describe a new method that uses the chimeric Galpha protein 16z25 to facilitate this process. Stably expressed in Chinese hamster ovary cells, 16z25 allows G(i/o)- and G(s)-coupled receptors to mobilize intracellular Ca(2+) upon agonist stimulation. We have generated nine cell lines each stably expressing 16z25 and a GPCR. All cell lines respond to appropriate agonist stimulation in fluorometric imaging plate reader (FLIPR) assays with robust and potent Ca(2+) mobilization. Several of these lines have been pharmacologically characterized using agonists and antagonists. We also demonstrate that the coexpression of GPCR and 16z25 does not interfere with the receptors' ability to activate endogenous signaling pathways. The ability of 16z25 to functionally mediate the agonist stimulation of a broad spectrum of GPCRs indicates that the use of cell lines stably coexpressing this chimera and GPCRs will simplify the drug screening process and aid in the deorphanization of new receptors.

Hypoxia induces differentiation of pulmonary artery adventitial fibroblasts into myofibroblasts

Activation of the alpha-smooth muscle actin (alpha-SMA) gene during the conversion of fibroblasts into myofibroblasts is an essential feature of various fibrotic conditions. Microvascular compromise and thus local environmental hypoxia are important components of the fibrotic response. The present study was thus undertaken to test the hypothesis that hypoxia can induce transdifferentiation of vascular fibroblasts into myofibroblasts and also to evaluate potential signaling mechanisms governing this process. We found that hypoxia significantly upregulates alpha-SMA protein levels in bovine pulmonary artery adventitial fibroblasts. Increased alpha-SMA expression is controlled at the transcriptional level because the alpha-SMA gene promoter activity, assayed via a luciferase reporter, was markedly increased in transfected fibroblasts exposed to hypoxia. Hypoxic induction of the alpha-SMA gene was mimicked by overexpression of constitutively active Galphai2 (alphai2Q205L) but not Galpha16 (alpha-16Q212L). Blockade of hypoxia-induced alpha-SMA expression with pertussis toxin, a Galphai antagonist, confirmed a role for Galphai in the hypoxia-induced transdifferentiation process. c-Jun NH2-terminal kinase (JNK) inhibitor II and SB202190, but not U0126, also attenuated alpha-SMA expression in hypoxic fibroblasts, suggesting the importance of JNK in the differentiation process. Hypoxia-induced increase in bromodeoxyuridine incorporation, which occurred concomitantly with hypoxia-induced differentiation, was blocked by U0126, suggesting that DNA synthesis and alpha-SMA expression take place through simultaneously activated parallel signaling pathways. Neutralizing antibody against transforming growth factor-beta1 blocked only 30% of the hypoxia-induced alpha-SMA promoter activity. Taken together, our results suggest that hypoxia induces differentiation of vascular fibroblasts into myofibroblasts by upregulating the expression of alpha-SMA, and this increase in alpha-SMA level occurs through Galphai- and JNK-dependent signaling pathways.

Constitutively active Galpha16 stimulates STAT3 via a c-Src/JAK- and ERK-dependent mechanism

The hematopoietic-specific Galpha16 protein has recently been shown to mediate receptor-induced activation of the signal transducer and activator of transcription 3 (STAT3). In the present study, we have delineated the mechanism by which Galpha16 stimulates STAT3 in human embryonic kidney 293 cells. A constitutively active Galpha16 mutant, Galpha16QL, stimulated STAT3-dependent luciferase activity as well as the phosphorylation of STAT3 at both Tyr705 and Ser727. Galpha16QL-induced STAT3 activation was enhanced by overexpression of extracellular signal-regulated kinase 1 (ERK1), but was inhibited by U0126, a Raf-1 inhibitor, and coexpression of the dominant negative mutants of Ras and Rac1. Inhibition of phospholipase Cbeta, protein kinase C, and calmodulin-dependent kinase II by their respective inhibitors also suppressed Galpha16QL-induced STAT3 activation. The involvement of tyrosine kinases such as c-Src and Janus kinase 2 and 3 (JAK2 and JAK3) in Galpha16QL-induced activation of STAT3 was illustrated by the combined use of selective inhibitors and dominant negative mutants. In contrast, c-Jun N-terminal kinase, p38 MAPK, RhoA, Cdc42, phosphatidylinositol 3-kinase, and the epidermal growth factor receptor did not appear to be required. Similar observations were obtained with human erythroleukemia cells, where STAT3 phosphorylation was stimulated by C5a in a PTX-insensitive manner. Collectively, these results highlight the important regulatory roles of the Ras/Raf/MEK/ERK and c-Src/JAK pathways on the stimulation of STAT3 by activated Galpha16. Demonstration of the involvement of different kinases in Galpha16QL-induced STAT3 activation supports the involvement of multiple signaling pathways in the regulation of transcription by G proteins.

Identification of tetratricopeptide repeat 1 as an adaptor protein that interacts with heterotrimeric G proteins and the small GTPase Ras

The biological functions of heterotrimeric G proteins and small GTPases are modulated by both extracellular stimuli and intracellular regulatory proteins. Using Saccharomyces cerevisiae two-hybrid screening, we identified tetratricopeptide repeat 1 (TPR1), a 292-amino-acid protein with three TPR motifs, as a Galpha16-binding protein. The interaction was confirmed both in vitro and in transfected mammalian cells, where TPR1 also binds to several other Galpha proteins. TPR1 was found to interact with Ha-Ras preferentially in its active form. Overexpression of TPR1 promotes accumulation of active Ras. TPR1 was found to compete with the Ras-binding domain (RBD) of Raf-1 for binding to the active Ras, suggesting that it may also compete with Ras GTPase-activating protein, thus contributing to the accumulation of GTP-bound Ras. Expression of Galpha16 strongly enhances the interaction between TPR1 and Ras. Removal of the TPR1 N-terminal 112 residues abolishes potentiation by Galpha16 while maintaining the interaction with Galpha16 and the ability to discriminate active Ras from wild-type Ras. We have also observed that LGN, a Galphai-interacting protein with seven TPR motifs, binds Ha-Ras. Thus, TPR1 is a novel adaptor protein for Ras and selected Galpha proteins that may be involved in protein-protein interaction relating to G-protein signaling.

Constitutive inositol phosphate formation in cytomegalovirus-infected human fibroblasts is due to expression of the chemokine receptor homologue pUS28

An open reading frame (ORF), US28, with homology to mammalian chemokine receptors has been identified in the genome of human cytomegalovirus (HCMV). Its protein product, pUS28, has been shown to bind several human CC chemokines, including RANTES, MCP-1, and MIP-1 alpha, and the CX(3)C chemokine fractalkine with high affinity. Addition of CC chemokines to cells expressing pUS28 was reported to cause a pertussis toxin-sensitive increase in the concentration of cytosolic free Ca(2+). Recently, pUS28 was shown to mediate constitutive, ligand-independent, and pertussis toxin-insensitive activation of phospholipase C via G(q/11)-dependent signaling pathways in transiently transfected COS-7 cells. Since these findings are not easily reconciled with the former observations, we analyzed the role of pUS28 in mediating CC chemokine activation of pertussis toxin-sensitive G proteins in cell membranes and phospholipase C in intact cells. The transmembrane signaling functions of pUS28 were studied in HCMV-infected cells rather than in cDNA-transfected cells. Since DNA sequence analysis of ORF US28 of different laboratory and clinical strains had revealed amino acid sequence differences in the amino-terminal portion of pUS28, we compared two laboratory HCMV strains, AD169 and Toledo, and one clinical strain, TB40/E. The results showed that infection of human fibroblasts with all three HCMV strains led to a vigorous, constitutively enhanced formation of inositol phosphates which was insensitive to pertussis toxin. This effect was critically dependent on the presence of the US28 ORF in the HCMV genome but was independent of the amino acid sequence divergence of the three HCMV strains investigated. The constitutive activity of pUS28 is not explained by expression of pUS28 at high density in HCMV-infected cells. The pUS28 ligands RANTES and MCP-1 failed to stimulate binding of guanosine 5'-O-(3-[(35)S]thiotriphosphate to membranes of HCMV-infected cells and did not enhance constitutive activation of phospholipase C in intact HCMV-infected cells. These findings raise the possibility that the effects of CC chemokines and pertussis toxin on G protein-mediated transmembrane signaling previously observed in HCMV-infected cells are either independent of or not directly mediated by the protein product of ORF US28.

Recent progress in alpha 1-adrenoceptor pharmacology

The adrenoceptors (ARs) play a key role in the modulation of sympathetic nervous system activity and are a site of action for many clinically important therapeutic agents. The alpha1-adrenoceptor subtypes (alpha1A-, alpha1B-, and alpha1D-AR) play a prominent role in regulating vascular tone and hypertrophic growth of smooth muscle and cardiac cells. Their functional characteristics with respect to ligand binding and second messenger utilization have been well described. Here, we review recent progress on subtype-specific subcellular localization, participation in signaling cascades, and the pivotal function of alpha1-ARs, as delineated through studies on genetically engineered animals. Together, these findings will provide new insights into the physiological and pathophysiological roles of the alpha1-ARs.

Gα16Couples Chemoattractant Receptors to NF-κB Activation

The guanine nucleotide-binding regulatory protein alpha-subunit, Galpha(16), is primarily expressed in hemopoietic cells, and interacts with a large number of seven-membrane span receptors including chemoattractant receptors. We investigated the biological functions resulting from Galpha(16) coupling of chemoattractant receptors in a transfected cell model system. HeLa cells expressing a kappaB-driven luciferase reporter, Galpha(16), and the formyl peptide receptor responded to fMLP with a approximately 7- to 10-fold increase in luciferase activity. This response was accompanied by phosphorylation of IkappaBalpha and elevation of nuclear kappaB-DNA binding activity, indicating activation of NF-kappaB. In contrast to Galpha(16), expression of Galpha(q), Galpha(13), and Galpha(i2) resulted in a marginal increase in kappaB luciferase activity. A GTPase-deficient, constitutively active Galpha(16) mutant (Q212L) could replace agonist stimulation for activation of NF-kappaB. Furthermore, expression of Galpha(16) (Q212L) markedly enhanced TNF-alpha-induced kappaB reporter activity. The Galpha(16)-mediated NF-kappaB activation was paralleled by an increase in phospholipase C-beta activity, and was blocked by pharmacological inhibitors of protein kinase C (PKC) and by buffering of intracellular Ca(2+). The involvement of a conventional PKC isoform was confirmed by the finding that expression of PKCalpha enhanced the effect of Galpha(16), and a dominant negative PKCalpha partially blocked Galpha(16)-mediated NF-kappaB activation. In addition to formyl peptide receptor, Galpha(16) also enhanced NF-kappaB activation by the C5a and C3a receptors, and by CXC chemokine receptor 2 and CCR8. These results suggest a potential role of Galpha(16) in transcriptional regulation downstream of chemoattractant receptors.

Involvement of c-Jun N-terminal kinase and p38 mitogen-activated protein kinase in alpha1B-adrenergic receptor/Galphaq-induced inhibition of cell proliferation

Certain G protein-coupled receptors (GPCRs) stimulate the activities of c-Jun N-terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK), members of the MAPK family. We investigated the role of JNK and p38 MAPK activation induced by the alpha1B-adrenergic receptor in the proliferation of human embryonic kidney 293T cells. Activation of the alpha1B-adrenergic receptor resulted in inhibition of cell proliferation. This receptor-induced inhibition of proliferation was blocked by a kinase-deficient MKK4 and by the p38 MAPK inhibitor SB203580. Additionally, transfection of constitutively activated Galphaq into cells also led to inhibition of proliferation in a JNK- and p38 MAPK-dependent manner. These results demonstrate that the alpha1B-adrenergic receptor/Galphaq signaling inhibits cell proliferation through pathways involving JNK and p38 MAPK.

Hematopoietic recovery after IEV chemotherapy for malignant lymphoma followed by different cytokines can be monitored by analysis of Galpha 16 and CD34

The hematopoiesis-specific G protein alpha subunit Galpha16 is a specific element in the signal transduction of the early hematopoietic cytokine network. As Galpha16 mRNA can be detected in early hematopoietic progenitor cells, RT-PCR for Galpha16 can be used as a sensitive marker of hematopoietic activity. The aim of this study was to test the possible use of Galpha16 determinations for monitoring cytokine effects on hematopoietic recovery after chemotherapy in patients. We correlated presence of Galpha16 mRNA and CD34 surface antigen with hematopoietic recovery in six lymphoma patients undergoing salvage therapy with different cytokine support (IEV followed by G-CSF, IL-3, or placebo). Regardless of different cytokine schedules with different time courses, hematopoietic recovery was always preceded by transcription of Galpha16. Monitoring the expression of Galpha16 mRNA by RT-PCR is a highly sensitive diagnostic tool for analyzing hematopoietic recovery after chemotherapy and for characterizing the effects of cytokines on hematopoiesis.

Interleukin-8 and other agonists of Gi2 proteins: autocrine paracrine growth factors for human hematopoietic progenitors acting in synergy with colony stimulating factors

We have reviewed the current knowledge on CXC chemokine interleukin-8 (IL-8) and human hematopoiesis, and more generally on agonists of heterotrimeric Gi2 proteins as regulators of human hematopoiesis. It appears that low doses of IL-8, a Gi2-agonist produced in an autocrine fashion by normal hematopoietic progenitors, mature blood cells and leukemic cells, promotes cell survival or/and proliferation in response to hematopoietic cytokines. More importantly, inactivation of the IL-8/Gi2 pathways inhibits CD34+ cell proliferation and colony formation. Similar positive effects on hematopoiesis of other, physiological or pathological, agonists of Gi2 proteins are discussed, as well as the molecular pathways involved and the consequences of activation of other G proteins (Gq, G16) by IL-8 and other Gi2-agonists.

Induction of erythroid differentiation by altered Galpha16 activity as detected by a reporter gene assay in MB-02 cells

Heterotrimeric G proteins may assume modulatory roles in cellular proliferation and differentiation. The G protein alpha-subunit Galpha16, which is specifically expressed in hematopoietic cells, is highly regulated during differentiation of normal and leukemic cells. In human erythroleukemia cells, suppression of Galpha16 inhibited cellular growth rates. A reporter gene system was established to assess the role of Galpha16 on erythroid differentiation of MB-02 erythroleukemia cells. It is based on transient transfection with a plasmid that expresses green fluorescent protein under the control of the beta-globin promoter. Expression of Galpha16 led to a significant increase in green fluorescent protein-positive cells, as did transfection with a Galpha16 antisense plasmid (154 and 156% of controls, respectively). The GTPase-deficient, constitutively active mutant of Galpha16, Galpha16R186C, further stimulated differentiation to 195% of control values. Because the effect of Galpha16 is triggered most efficiently by the GTP-bound protein, an indirect action through interference of overexpressed Galpha16 with G protein betagamma-subunits can be excluded. The corresponding mutant of Galphaq (GalphaqR182C), the phylogenetically closest family member of Galpha16, had no effect. The data define a specific role for Galpha16-dependent signal transduction in cellular differentiation: deviations from optimal levels of Galpha16 functional activity lead to reduced growth rates and promote differentiation in hematopoietic cells.

Inhibition of cell differentiation by G alpha q in the renal epithelial cell line LLC-PK1

LLC-PK1, an epithelial cell line derived from the kidney proximal tubule, was used to study the ability of the G protein alpha-subunit, G alpha q, to regulate cell differentiation. A constitutively active mutant protein, alpha qQ209L, was expressed using the LacSwitch-inducible mammalian expression system. Induction of alpha qQ209L expression with isopropyl-beta-D-thiogalactopyranoside (IPTG) enhanced phospholipase C activity maximally by 6- to 7.5-fold. Increasing concentrations of IPTG progressively inhibited the activity of two differentiation markers, Na(+)-dependent hexose transport and alkaline phosphatase activity. Induction of alpha qQ209L expression also caused a change from an epithelial to a spindle-shaped morphology. The effects of alpha qQ209L expression on cell differentiation were similar to those observed with 12-O-tetradecanoylphorbol 13-acetate (TPA) treatment. However, protein kinase C (PKC) levels were downregulated in TPA-treated cells but not in alpha qQ209L-expressing cells, suggesting that the regulation of PKC by G alpha q may be different from regulation by TPA. Interestingly, the PKC inhibitor GF-109203X did not inhibit the effect of IPTG on the development of Na(+)-dependent hexose transport in alpha qQ209L-expressing cells. These data implicate PKC delta and PKC epsilon in the pathway used by G alpha q to block the development of Na(+)-dependent hexose transport in IPTG-treated cells.

Galpha16 mimics vasoconstrictor action to induce smooth muscle alpha-actin in vascular smooth muscle cells through a Jun-NH2-terminal kinase-dependent pathway

Prolonged exposure of vascular smooth muscle cells (VSMC) to vasoconstrictors such as vasopressin or angiotensin II induces hypertrophy and increases expression of muscle-specific genes including smooth muscle alpha-actin (SM-alpha-actin). These vasoconstrictors signal through G-proteins, including members of the Gq family. To further investigate the role of Gq family members, VSMC were transfected with a constitutively active mutant of a Gq family member, Galpha16 (Galpha16Q212L). Stable expression of Galpha16Q212L persistently stimulated phospholipase C, resulting in increased basal levels of inositol phosphates. These cells were hypertrophied and expressed elevated levels of SM-alpha-actin compared with wild-type VSMC or cells transfected with a control plasmid (Neo). SM-alpha-actin promoter activity was markedly increased in cells stably or transiently expressing Galpha16Q212L. Basal c-Jun-NH2-terminal kinase (JNK) activity was increased 3-9-fold in cells stably expressing Galpha16Q212L, while basal activity of the p42/44 mitogen-activated protein kinases (ERKs) was unaffected. Transient expression of a kinase inactive JNK kinase partially inhibited induction of SM-alpha-actin promoter activity in response to vasoconstrictors or expression of Galpha16Q212L. These results indicate that expression of constitutively active Galpha16 in VSMC mimics the effects of vasoconstrictors on hypertrophy and muscle-specific gene expression, and activation of JNK may play a role in these responses.

Platelet-activating factor (PAF) induces growth stimulation, inhibition, and suppression of oncogenic transformation in NRK cells overexpressing the PAF receptor

Platelet-activating factor (PAF) is a phospholipid mediator with various physiological functions, including cellular growth and transformation. PAF exerts biological activities through G-protein-coupled receptors. In normal rat fibroblasts overexpressing a cloned PAF receptor, PAF induced immediate early oncogene expression and mitogenic responses. On the other hand, PAF strongly inhibited the epidermal growth factor-induced mitogenic growth response, growth acceleration, and anchorage-independent cell growth in a soft agar. Furthermore, PAF suppressed v-src- or v-ras-induced oncogenic morphological changes and anchorage-independent growth. Our observations suggest that PAF is a unique growth regulator with apparently diverse functions. Dual actions of PAF may relate to the point of action in the cell cycle; PAF stimulates the mitogenic response in G0-arrested cells in a pertussis toxin-sensitive manner, while it inhibits the G1 to S transition through a pertussis toxin-resistant manner.

The P2U purinoceptor obligatorily engages the heterotrimeric G protein G16 to mobilize intracellular Ca2+ in human erythroleukemia cells

To assess the role of G16, a trimeric G protein exclusively expressed in hematopoietic cells, Galpha16 antisense RNA was stably expressed in human erythroleukemia (HEL) cells. Western blot analysis showed that in transfected cell lines, the expression of endogenous Galpha16 protein was suppressed, but the expression of Galphaq/11, Galphai2, and Galphai3 remained unaffected. Suppression of Galpha16 in transfected HEL cells did not interfere with transient elevations of intracellular free Ca2+ concentrations induced by prostaglandin E1 (PGE1), platelet-activating factor, or thrombin. In parental HEL cells, UTP and ATP mobilized Ca2+ from intracellular stores with half-maximum effective concentrations of 3. 6 +/- 0.7 and 4.7 +/- 1.6 microM, respectively, apparently by stimulating P2U purinoceptors. By contrast, Ca2+ mobilization by UTP or ATP was completely abrogated in Galpha16-suppressed cells, indicating specific coupling of G16 to P2U purinoceptors. Pertussis toxin inhibited the effect of UTP in parental HEL cells by 57.6 +/- 4.9%. These data indicate that signaling by the P2U purinoceptor obligatorily requires G16 but may be modulated further by activation of Gi. Priming of HEL cells with UTP or ATP prior to stimulation with PGE1 markedly enhanced the PGE1-induced intracellular Ca2+ release. This indirect, potentiating effect of UTP and ATP was not impaired in Galpha16-suppressed cells but was inhibited by pertussis toxin, indicating that functional P2U purinoceptors are present on these cells and that the potentiating effect primarily depends on Gi. The data demonstrate (i) that Galpha16 antisense RNA selectively inhibits endogenous Galpha16 protein expression in HEL cells; (ii) that stimulation of endogenous P2U (P2Y2) purinoceptors leads to the mobilization of intracellular Ca2+ by a mechanism that strictly depends on Galpha16; and (iii) that P2U purinoceptors in HEL cells can communicate with two distinct signaling pathways diverging at the G protein level.

Discordant signal transduction and growth inhibition of small cell lung carcinomas induced by expression of GTPase-deficient G alpha 16

Small cell lung carcinoma (SCLC) accounts for 20-25% of primary lung cancers and is rapidly growing, widely metastatic, and rarely curable. Autocrine stimulation of multiple G protein-coupled neuropeptide receptor systems contributes to the transformed growth of SCLC. The ability of neuropeptide receptors to stimulate phospholipase C and mobilize intracellular Ca2+ indicates that Gq family members of heterotrimeric G proteins are a convergence point mediating autocrine signaling by multiple neuropeptides in SCLC. Expression of a GTPase-deficient, constitutive active form of an alpha q family member, alpha 16Q212L, in SCLC markedly inhibited growth of the cells in soft agar and tumor formation in nude mice. SCLC lines expressing alpha 16Q212L exhibited 2-4-fold elevated basal phospholipase C activity, but neuropeptide and hormone-regulated intracellular Ca2+ mobilization was nearly abolished. The data suggest that Ca2+ mobilization is an obligatory signal in neuropeptide-stimulated growth of SCLC. In addition, the proline-directed c-Jun NH2-terminal kinases/stress-activated protein kinases, which are members of the mitogen-activated protein kinase family, were stimulated approximately 2-fold in parental SCLC in response to exogenous neuropeptides and muscarinic agonists and were constitutively activated to the same degree in alpha 16Q212L-expressing SCLC. Thus, alpha 16Q212L expression induced desensitizaton of neuropeptide-stimulated Ca2+ signaling and persistent activation of the c-Jun NH2-terminal kinase/stress-activated protein kinase pathway. We propose that the induction of discordant signaling by selective perturbation of receptor-regulated effector systems leads to the inhibition of SCLC cell growth.

Endocrine disorders associated with mutations in guanine nucleotide binding proteins

The basis for a number of relatively rare endocrine diseases, which present clinically with features of AHO, have been shown conclusively to result from mutations in the G3 alpha gene which interfere with the expression of functional protein. Individual kindreds display a range of specific mutations in this gene. A further series of disorders result from somatic mutations of the G3 alpha gene which result in constitutive activation (in one case probably with a concomitant decrease in stability of the expressed protein). When such a mutation occurs in early embryogenesis it can result in a pattern of mosaicism of expression of clinical features in the patient. Despite these cases, equivalent alterations in other G-protein alpha subunit genes seem to be of limited importance in human disease. This is despite biochemical data from a range of experimental cell models which indicate that such mutations can have potent effects on cell growth and division.

Phosphoinositide-specific phospholipase C and mitogenic signaling

The importance of PLC activation in cell proliferation is evident from the fact that the hydrolysis of PtdIns(4,5)P2 is one of the early events that follow the interaction of many growth factors and mitogens with their respective receptors. However, the importance of PLC activation is not restricted to proliferation; it is one of the most common transmembrane signaling events elicited by receptors that regulate many other cellular processes, including differentiation, metabolism, secretion, contraction, and sensory perception. It is also clear that cell proliferation signaling does not always require PLC, as indicated by the fact that growth factors such as insulin and CSF-1 do not appear to elicit the hydrolysis of PtdIns(4,5)P2, even though the intracellular domains of their receptors carry a PTK domain and the receptors show topologies very similar to those of the PLC-activating growth factors PDGF, EGF, and FGF. The growth factor-dependent activation of PLC is initiated by the formation of a complex between the receptor PTK and PLC-gamma; the formation of this complex is mediated by a specific interaction between a tyrosine phosphate residue on the intracellular domain of PTK and the SH2 domain of PLC-gamma. The receptor PTK subsequently phosphorylates PLC-gamma, of which two distinct isozymes, PLC-gamma 1 and PLC-gamma 2, have been identified. Proliferation of T cells and B cells in response to the aggregation of their respective cell surface receptors is also accompanied by the activation of PLC-gamma isozymes at an early stage. Unlike growth factor receptors, the T cell and B cell receptors lack intrinsic PTK activity but associate with several non-receptor PTKs of the Src and Syk families. Although the specific kinases are not known, one or more of these enzymes phosphorylate and activate PLC-gamma 1 and PLC-gamma 2. Transduction of growth signals by G protein-coupled receptors such as those for thrombin or bombesin also requires PtdIns(4,5)P2 hydrolysis, which, in this instance, is mediated by PLC-beta isozymes. The PLC-beta subfamily consists of four distinct members: PLC-beta 1, PLC-beta 2, PLC-beta 3, and PLC-beta 4. Agonist interaction with specific G protein-coupled receptors causes the dissociation of Gq proteins into G alpha and G beta gamma subunits and the exchange of GDP bound to G alpha for GTP. The resulting GTP-bound G alpha subunit then activates PLC-beta isoforms by binding to the carboxyl-terminal region of the enzyme.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential modulation of bombesin-stimulated phospholipase C beta and mitogen-activated protein kinase activity by [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P

Mitogenic stimulation of Swiss 3T3 fibroblasts with bombesin results in receptor-mediated activation of a complex array of effectors, including phospholipase C beta and mitogen-activated protein (MAP) kinase. Incubation of Swiss 3T3 fibroblasts with the 11-amino acid [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P peptide inhibited bombesin-stimulated cell proliferation and phospholipase C beta activation even at high bombesin concentrations. The peptide did not inhibit the activation of phospholipase C beta by a GTPase-deficient form of the Gq-like protein, G16, indicating that the peptide does not inhibit phospholipase C beta and is acting at a point upstream of the activated form of the G protein alpha subunit. The peptide inhibited MAP kinase activation at low bombesin concentrations, but unlike phospholipase C beta, this inhibition could be overcome with 30 nM bombesin. In control Swiss 3T3 cells, bombesin did not measurably activate Ras or Raf-1 above basal levels. Following incubation of the cells with the [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P peptide, 50 nM bombesin activated Raf-1 4-6-fold over basal levels. Platelet-derived growth factor-stimulated activities of PLC, Ras, Raf-1, and MAP kinase were unaltered after incubation of Swiss 3T3 cells with the [D-Arg1,D-Phe5,D-Trp7,9,Leu11]substance P peptide, as was platelet-derived growth factor-stimulated growth of the Swiss 3T3 cells. Thus, the peptide behaves as an antagonist that differentially inhibited phospholipase C beta and MAP kinase signal transduction pathways. The growth arrest observed with the peptide indicates that the bombesin-stimulated activation of MAP kinase is not sufficient to support mitogenesis in Swiss 3T3 cells.

Gq pathway desensitizes chemotactic receptor-induced calcium signaling via inositol trisphosphate receptor down-regulation

Desensitization of a chemotactic receptor is an adaptive process that terminates inflammation. Although homologous desensitization can be well explained by the action of specific receptor kinases, the mechanisms of heterologous desensitization remain elusive. As an approach to evaluate the roles of Gq pathway in desensitization of calcium signaling, we expressed a constitutively active Gq alpha mutant (Gq alpha Q-L) together with platelet-activating factor (PAF) receptor in Xenopus laevis oocytes. Gq alpha Q-L expression completely attenuated the calcium-sensitive chloride current and the 45Ca release elicited by PAF. The Gq-mediated desensitization could not be ascribed to G protein/receptor uncoupling via receptor phosphorylation, because (i) PAF-induced inositol 1,4,5-trisphosphate (IP3) synthesis was only partially suppressed and (ii) a mutated PAF receptor devoid of all Ser and Thr in the third cytoplasmic loop and in the C-terminal tail was also completely desensitized by Gq alpha Q-L. In Gq alpha Q-L expressing oocytes, microinjection of IP3 failed to elicit the calcium response, and the IP3 receptor, detected by a specific antibody, disappeared. Thus, the Gq-mediated desensitization can be most likely explained by IP3 receptor down-regulation. These novel mechanisms may explain in part heterologous desensitization in chemotactic factor-stimulated inflammatory cells.